Described below is a digital logic unit which can be reconfigured in nonvolatile form having cells which have a magnetic layer system and whose resistance can be altered by magnetic field pulses. Included in the digital logic unit are a first line path containing series-connected data cells and a second line path containing series-connected configurable configuration cells and a logic unit for evaluation of the configured states.
These cells allow information to be stored on a magnetic basis. The magnetization of a layer of the cell's magnetic layer system can be changed by a magnetic field pulse, which changes the magnetoresistance of this layer by a few percent. The respective resistance can be read and is a measure of the logic state of the cells.
It has already been proposed to implement programmable logic functions with cells which make use of the GMR effect. Such logic circuits may be of similar design to known programmable logic arrays and use magnetoresistive cells as nonvolatile programmable elements or as arrays of logic gates, the logic functions being able to be programmed by magnetic logic. These logic circuits have the advantage that both the program information and the data are nonvolatile. It is therefore possible to reprogram an entire logic array to a particular configuration as required.
The article “Programmable logic using giant-magnetoresistance and spin-dependent tunneling devices” (Black and Das, Journal of Applied Physics, Vol. 87, No. 9, Jan. 5, 2000) proposes magnetic comparator logic with a first line path containing series-connected data cells and a second parallel data path containing series-connected configuration cells. The possible states are evaluated by comparing configured magnetic cell contents with data cells. Both paths, that is to say the data cells and the configuration cells, carry a prescribed constant current, and the voltage drops produced by the respective cells are supplied to the inputs of a comparator. The magnetic configuration of the individual cells, which have either a high or a low resistance, produces a stepped signal profile. At the lowest voltage value, all the resistors have a low value, and if one has a high value and all the others have a low value then the next level is produced. To be able to evaluate the configured states, a comparator is required which ascertains the difference voltage for the two parallel paths. The comparator is able to check whether a particular condition is met; by way of example, the condition may be: “resistance of the path for the data cells is less than or equal to the resistance of the path for the configuration cells”. Stipulating the switching thresholds is difficult, however, since a series of parameters influences the properties of the magnetic cells. By way of example, these include the inequality of current sources in the two paths, fluctuations caused by lithography, discrepancies in the supply resistors, any asymmetric response which the comparator may have, and other effects.